Control system for refrigeration compressor

ABSTRACT

A refrigeration control system for a closed vapor cycle refrigeration circuit includes means for sensing the inlet temperature and outlet temperature of the evaporator coil to provide a superheat signal. The temperature in the controlled space is compared against a set point temperature to provide a control signal. The superheat and control signals are compared to produce a modulating signal for regulating the compressor operation. A trigger circuit overrides the superheat signal when the system is started, at which time there is no temperature difference across the evaporator coil. A limit circuit overrides the control signal to provide zero capacity output of the system when the superheat equals zero.

United States Patent 1191 J ednacz et al.

[ Apr. 16, 1974 CONTROL SYSTEM FOR REFRIGERATION COMPRESSOR Primary Examiner-Meyer Perlin [75] Inventors: Thomas C. Jednacz; Dean K. Attorney Agent or Flrm Donald Banner P Norbeck, both of York, a I ABSTRACT [73 1 Asslgnee: Bm'g'wamer corporatlon chlcago A refrigeration control system for a closed vapor cycle 1 refrigeration circuit includes means for sensing the [22] Fil d; J 20, 1972 inlet temperature and outlet temperature of the evaporator coil to provide a superheat signal. The tempera- [211 Appl 264686 ture in the controlled space is compared against a set point temperature to provide a control signal. The su- [52 US. Cl. 62/209, 62/227 porhoat and control signals are compared to produce a 51 int. Cl F25b 1/00 modulating signal for regulating the pr r p r- [58] Field of Search 62/203, 208, 213, 215, ation- A trigger circuit overrides the suporhoat signal 62/227 209 when the system is started, at which time there is no temperature difference across the evaporator coil. A [56] References Cited limit circuit overrides the control signal to provide UNITED STATES PATENTS zero capacity output of the system when the superheat 2,524,813 10/1950 Lathrop., 62/213 equals Zero 3,335,576 8/1967 Phillips 62/156 8 Claims, 3 Drawing Figures 7 Ex pension D e\/ ice 17 KN Inlet ,-y' Temp. 16 I b v/ a Evaporator Cordenser ll 33 i 32 4 13 Outle.

Te rr. .Z 2 Set 1 D 5 2p MOiOI" 2 e t Systerrv Irv r 1 A 18 Cor. 1 pressor 1C3volts DC 24 2O m I Cor VJ-GQQ 12vo|ts D-C Temp. Regulator" CONTROL SYSTEM FOR REFRIGERATION COMPRESSOR BACKGROUND OF THE INVENTION Various systems have been devised for air conditioning applications useful with automotive vehicles, such as cars and trucks. A difficult problem in such systems is the high temperature of the enclosed vehicle space, which may reach 180 F. with the vehicle parked, windows closed and the air conditioning off. After start-up and when the vehicle space has been reduced to a normal temperature level, approximately 75 F., the danger of freeze-up of the evaporator coil is present. To avoid this the pressure in the evaporator coil is increased and maintained at a constant temperature during subsequent system operation.

In one improved system particularly useful in this automotive environment, a valve is interposed in the suction line between the evaporator and the compressor inlet which is operable to control the flow of refrigerant from the evaporator. At the same time, the pressure in this line is used to control the setting of a constant pressure expansion device feeding refrigerantto the evaporator. The valve in the suction line is set to produce about a lb. pressure drop when the diaphragm chamber is at atmospheric pressure. A vacuum is then applied to the diaphragm chamber at a controlled rate which has the effect of opening up the valve and raising the pressure in the suction line downstream from the valve. This pressure is used as a pneumatic control signal to the constant pressure expansion valve to raise the control point at which the evaporator pressure is controlled. This. improved systernis disclosed and claimed in the copending application of A. B. Newton, entitled Refrigeration Control System, having Ser. No. 264,649, filed concurrently with the present application.

The present invention is. particularly directed to a control system useful to regulate systems of the type set out in the copending application.'lt is a principal consideration to provide such a system whicheffectively measures both the superheat and thetemperature differe nce between the. set point and the actual space in the automotive vehicle, and' modulates the compressor operation in accordance with these superheat and temperature difference signals.

Another important consideration of the invention is the provision of an override arrangement during system start-up when there is no temperature difference across the evaporator coil.

Yet another important consideration is the provision of some means for limiting the temperature difference signal so that at a certain relationship of the space temperature to the set'point temperature, there will be a zero capacity output of the compressor when the superheat signal is zero.

SUMMARY OFYTHE: INVENTION section to provide a first signal which varies as a function of the evaporator inlet temperature. A second sensor is positioned adjacent the evaporator coil output portion to provide a second signal which varies as the evaporator outlet temperature. Means, such as an integrated circuit connected as a differential amplifier, is provided to combine these first and second signals and produce a superheat signal which varies in accordance with the temperature difference between the inlet and outlet portions of the evaporator coil.

A second circuit includes an adjusting means, such as a potentiometer, for establishing the temperature set point for the controlled space. A third sensor is disposed in this space, for providing a third signal which varies as a function of the actual temperature in the controlled space. Means, such as another operational amplifier, is provided to combine the set-point signal with the actual space temperature signal, thus producing a control signal which varies as a function of the temperature difference between the set point and the actual space temperature.

A comparator stage has a first input connection for receiving the superheat signal and a second input connection for receiving the control signal. The comparator stage output connection provides the modulating signal required to regulate the compressor operation and control the temperature of the space as a function of the difference between the superheat and control signals.

In accordance with another feature of the invention, the control system includes a resistor coupled in series with a capacitor. This series circuit is coupled between the first input connection and the output connection of the comparator circuit. The capacitor has an integrating function for the comparator stage. A diode is coupled in parallel with the resistor to modify the charging anddischarging times of theRC circuit in the proper direction to decelerate the change in the modulating signal as the superheat increases, compared to the rate of change in the modulating signal when the superheat decreases.

Yet another feature for the invention is the-incorporation of a trigger circuit in, the control system, which trigger circuit includes a first input connection connected to receive the signal related to the evaporator coil inlet temperature provided by the first sensor, and a second input connection which receives a reference signal. The output connection of this trigger circuit is coupled to the first input connection of the integratorcomparator stage, thus effectively overriding the superheat signal during start-up when there is no temperature difference across the evaporator coil.

In accordance with another aspect of the invention, a limiter stage is connected with its first input connected to receive the control signal, and a second input connection for receiving a reference signal. The output connection of the limiter stage applies an override signal to the second input connection of the signal combining stage in the second circuit, to override the control signal and thus allow a zero capacity output indication when the superheat signal indicates zero superheat- THE DRAWINGS In the several figures 0f the drawings, like reference numerals identify like components, and in the draw ings:

FIG. 1 is a block diagram of a closed vapor cycle refrigeration circuit including the control system of this invention;

FIG. 2 is a schematic diagram setting out circuit details of the control system of this invention; and

FIG. 3 is a graphical illustration useful in understanding operation of the invention.

GENERAL SYSTEM DESCRIPTION FIG. 1 is a general showing of a closed vapor cycle refrigeration circuit. This circuit includes an evaporator coil coupled over a suction line 11 to the return side of compressor 12, which is also coupled over line 13 to one side of condenser 14. The upper side of condenser 14 is coupled through line 15 to the inlet side of an expansion device 16, the output side of which is connected through a line 17 to the inlet portion of evaporator coil 10. A motor 18 is mechanically coupled over a belt 20 to drive compressor 12 when the motor receives electrical energy over line 21 from inverter 22. Of course any suitable arrangement can be utilized to drive the compressor, and other sources of electrical energy can be substituted for inverter 22. The present invention is particularly useful in connection with an automotive air conditioning system, in which the'12 volts d-c energy from the car or truck battery is provided on line 23, and applied over line 24 to energize inverter 22.

In accordance with the present invention, a control system 25 is provided and connected to produce a modulating signal for application over line 26 to regulate the frequency of the a-c energy produced by the inverter, thus regulating the speed of motor 18 and the effective capacity of compressor 12. Of course this is just one system for utilizing the modulating signal on line 26 to regulate the capacity of the compressor. The modulating signal could also be used to regulate the operation of a valve, or some other device, for regulating the capacity of compressor 12.

a voltage regulator 27 can be provided as shown to establish a regulated voltage level over line 28 for energizing control system 25. In one embodiment this regulator was effective to provide a well-regulated 10 volts level on line 28. Various configurations of voltage regulators are well known and understood in this art, and the details therefore will not be set out herein.

The control system 25 of this invention receives a first signal over line 30 from a first sensor 31, positioned at or adjacent the inlet side of evaporator coil 10. A second signal is received over line 32 from a second sensor 33 positioned adjacent the evaporator coil outlet portion; hence the second signal varies as the evaporator outlet temperature. Control system 25 also receives a temperature set-point signal over line 34 from an adjusting means 35, shown as a simple potentiometer. The system also receives a third signal over line 36 from a third sensor 37 positioned in the controlled space, such as the inside of the car or truck at a location near the dashboard. Thus this third signal on line 36 varies as a function of the actual space temperature. Although it is apparent that the sensors 31, 33, 37 and the adjusting unit may not be positioned within the same physical enclosure as the components shown in FIG. 2, because of their interelationship with the control system they will be shown as portions of that system in FIG. 2 and so described in the specification and claims.

DETAILED DESCRIPTION OF THE INVENTION In the lower left portion of FIG. 2 first and second sensors 31, 33 are depicted as thermistors, each having one side connected to a common conductor 40 to receive energy over a resistor 41 from line 28. The first signal, from first sensor 31, is passed over line 30 and resistor 42 to the first input connection of the operational amplifier 43. The second signal, from second sensor 33, is passed over line 32 and resistor 44 to the second input connection of the same operational amplifier (op amp). The output connection of this amplifier is coupled over a feedback resistor 45 to the second input connection. Thus stage 43 combines the first and second signals and provides a superheat signal from its output terminal over conductor 46. Op amp 43 can be one half of the two circuits in the standard line of Texas Instrument integrated circuits. This amplifier and op amp 47, in the second circuit which will be described below, can be located in the same package. In the same way a single integrated circuit (IC) can provide the additional stages 48, 50, and a third circuit provides the limiter circuit 51.

Operational amplifier 43 is one means for combining the first signal on line 30 with the second signal on line 32 to provide the superheat signal on line 46, which varies as a function of the temperature difference between the inlet and outlet portions of the evaporator. Other combining circuits or op amp circuits can be substituted so long as the superheat signal is produced on conductor 46. In the illustrated circuit terminal 4 of the op amp is grounded, and input terminal 5 is connected in a voltage divider circuit including conductor 28, resistor 52, tenninal 5, and resistor 53 to ground. Those skilled in the art will recognize that this connection of pin 5 in the voltage divider circuit 52, 53, together with the negative feedback provided over resistor 45 in this circuit, makes the operational amplifier circuit 43 a true differential amplifier. Comparator-integrator stage is likewise connected in a differential amplifier circuit. The pin connections for the various op amp stages are shown to facilitate practice of the invention with a minimum of experimentation.

In the second circuit of the control system, adjusting means 35 provides a temperature set-point signal which is applied over line 34 and resistor 54 to one input connection of operational amplifier 47. Third sensor 37 provides a third signal the actual space temperature signal which is applied over line 36 and resistor 55 to the other input terminal of op amp 47. A resistor 56 and a capacitor 57 are connected in parallel between the 2 input terminal and the output connection of op amp 47 to provide noise immunity for this circuit. The eight pin connection of amplifier 47 is coupled to conductor 28. Amplifier 47 is a means for combining the set-point signal with the third signal from sensor 37 representing the actual space temperature, for providing on conductor 58 a control signal which varies as a function of the difference between the set-point temperature and the actual space condition.

Another operational amplifier 50 is connected as a comparator stage to receive the control signal and superheat signal, and provide on line 26 a modulating signal for regulating the compressor operation. The control signal is applied from amplifier 47 over conductor 58 and resistor 60 to the 5 input connection of amplifier 50. A voltage divider circuit includes conductor 28 and a pair of resistors 61, 62, with the lower end of resistor 62 being grounded. Input connection 5 is coupled to the common connection between resistors 61, 62 in this voltage divider arrangement, making the circuit including op amp 50 a true differential amplifier.

The superheat signal on line 46 is applied over resistor 63 and conductor'64 to the 6 input connection of op amp 50. This amplifier compares the superheat signal with the control signal to provide a resultant modulating signal on line 26 to control the compressor operation, as will be described hereinafter in connection with FIG. .3.

Connection between input connection 6 and the output terminal 7 of comparator stage 50 is a series RC circuit including a resistor 65 and a capacitor 66. Capacitor 66 provides an integrating function so that stage 50 becomes an integrator-comparator stage. Resistor 67 is coupled in parallel with this RC circuit, and a unidirectional current conduction means, shown as a diode 68, is coupled in parallel with resistor 65. Terminal 4 stage 50 is grounded. The purpose of diode 68 is to reduce the rate at which compressor capacity is decreased with lower end of resistor 80 iscoupled over resistor 77 to ground; over another resistor 81 to conductor 32; and over a series circuit resistor 82 and the effective portion of variable resistor 83 to conductor 30.

The same control signal which appears on conductor 58 is applied over conductor 84 to the first input connection of another operational amplifier 51 connected as a limiter stage. The 7 pin connection of this op amp is connected to conductor 28, its 4 pin is grounded. and its number 6 or output connection is connected both over a diode 85 to the 2 input connection of op amp 47, and over a feedback capacitor 86 to the 2 input connection of the same stage 51. The 2 input connection is also coupled over conductor 87 to the movable tap of a potentiometer 88, connected between resistors 90 and 91 in an obvious voltage divider arrangement. This potentiometer 88 in its adjustment provides a reference signal for the limiter stage. Stage 51 provides an override signal which is applied over diode 85 to the second input connection of the signal combining means 47.

- The setting of potentiometer 88 is established such a decrease in amplitude of the superheat signal, as compared to the rate at which compressor capacity is increased when the superheat signal on line 46 increases. As will be shown in the operational description below, the superheat signal level on line 46 is inversely proportional to the value of the superheat as measured across the evaporator coil. Thus a decrease in the actual superheat produces an increase in the superheat signal, and, capacitor 66 is rapidly charged over diode 68 which in effect bypasses resistor 65. Thus the compressor capacity is rapidly decreased (with the decrease in amplitude of the modulating signal on line 26). However with an increase in the value of superheat across the coil, there is a decrease in the amplitude of the superheat signal on line 46; the discharge path for capacitor 46 includes resistor65, thus retarding the rate at which the compressor capacity is increased for a given amplitude change in the superheat signalcaused by a superheat increase.

Stage 48 is connectedas a trigger circuit to effect proper starting of the system. This is necessary because when the system has been shut down, there isno temperature difference across the evaporator coil to provide a superheat signal. Thus input connection 2 of op amp 48 is coupled over resistor 70 to line 30, which re ceives the signal connoting the evaporator input temperature from thermistor 31. The 3 input terminal of amplifier 48 iscoupled over resistors 71, 72 and 41 to conductor 28, thus providing a reference signal at the 2 input terminal. The 8 terminal of stage 48 is coupled to conductor 28. The output terminal of stage 48 is coupled over a feedback resistor 73 to the 3 input connection, and the output terminal is also coupled over diode 74 and a resistor 75 to the 6 input connection of integrator-comparator stage 50. Thus at start-up the reference signalat input terminal 3 is effective, through op am 48, to override the superheat signal when there is no temperature difference between the sensors at each end of the evaporator coil.

Thermistor 37 is coupled in a voltage divider circuit which includes conductor 28, resistors 41, 76, thermistor 37, and another resistor 77 to ground. Potentiometer 35 is coupled in the illustrated voltage divider circuit between resistors 78 and 80. The upper end of resistor 78 is coupled to an end of resistor 72, and the that, when the output of amplifier stage 47 indicates the space temperature is 25 F. above the set point, the limiting action is effective to allow the control system to produce zero capacity output from the compressor when the superheat signal is equal to zero.

In considering the operation of the control system shown in FIG. 2, it is desired to provide an increase in the throttling voltage (.the modulating signal on line 26) and thus increase the compressor capacity, at a rate which is slower than the rate at which capacity is decreased. This difference in capacity change rates is achieved by connection of diode 68 in parallel with resistor 65 to modify the charge and discharge rates of I capacitor 66. It should be noted that the amount of su perheat is indicated by the difference in temperatures sensed by thermistors 31 and 33. The differential amplifier stage 43 is connected so that its output signal, is

4.0 volts when there is a 30 F. superheat, the signal is 6.0 volts for 3.0 superheat, and the superheat signal is 6.2 volts when there is zero degree superheat. Thus the superheat signal amplitude is inversely proportional to the amount of superheat, and thus must be considered to provide the proper connection of diode 68 to make the increase in capacity occur more slowly for a superheat increase than does the decrease in capacity for a superheat decrease.

In operation the circuit including op amp 47 provides an output (control) signal of 4.0 volts when the car temperature is 2.5" F. below the set point, and a control signal of 6.0 volts when the space temperature exceeds the set point value by 25 F. The output signal level of amplifier.47 is limited by stage 51 at the 6 volts level, indicating a space temperature of 2.5" F. above the set point. This connection allows the control system to have zero capacity output when the superheat is zero.

The modulating signal provided by comparatorintegrator stage 50 was set to provide a 2.0 volts signal for zero capacity, and 8.0 volts for full capacity. The system gain is adjusted so that a change in superheat of 3, with reference to the controlled space temperature and to the set point value, will change the capacity from zero to full. The effect of this gain adjustment is that the system produces a three degree throttling range on superheat modulated by the space (vehicle) temperature. This is shown in FIG. 3. Curve 93 indicates the capacity changes from zero to full for a superheat change of 3, from 1.5 to 45, when the car temperature is 2.5 above the set point. With the car temperature at the set point, the same capacity change is realized as the superheat varies through 3, from to 18, shown by curve 94. Curve 95 indicates that when the car temperature is 2.5 below the set point, the capacity increases from zero to half over a throttling range of approximately 1.5 F.

The trigger circuit 48 function in the correct starting of the system has been described. This stage was connected to switch to 2.0 volts when the evaporator inlet temperature (sensed by thermistor 31) goes above 55 F., and to 8.0 volts when the evaporator inlet temperature goes below 50 F.

The circuit shown in FIG. 2 operated satisfactorily using op amps of the Texas Instruments series, with 10 volts d-c applied to all the conductors referenced 28. Op amps 43, 47 were provided by a TI type SN72558P device, and stages 48, 50 were identical. Limiter stage 51 was a TI SN72741P device. Thermistors 31, 33 were of the Fenwal UUA33J1 type, and thermistor 37 was a Fenwal KA31L4 unit. Circuit values for the other components are set out below. It is understood that these identifications and values are given only to facilitate implementation of the invention, and are in no sense a constraint upon the concept or circuit arrangement of the inventive system.

Component Identification or Value 6 ,74, 85 BAX18 66 2.0 uf 200 v. 86 0.005 uf 35 lo ohms 77 5| ohms 72 41.2 ohms 73 5.l V M 76 L180 ohms 78 9.l ohms 8 0 47.5 ohms 8] 5,760 ohms 82 9,090 ohms 83 500 ohms 88 I00 ohms 0 360 ohms 91 560 ohms While only a particular embodiment of the invention has been described and illustrated, it is apparent that various modifications and alterations may be made therein. It is therefore the intention in the appended claims to cover all such modifications and alterations as may fall within the true spirit and scope of the invention.

What is claimed is:

1. A refrigeration system including an evaporator, a compressor, a condenser and an expansion device, all intercoupled in a closed vapor cycle refrigeration circuit, with a control system for providing a modulating signal to regulate compressor operation, which control system comprises:

a first circuit, including a first sensor positioned adjacent the evaporator input to provide a first signal which varies as a function of the evaporator inlet temperature, a second sensor positioned adjacent the evaporator output to provide a second signal which varies as the evaporator outlet temperature, and means for combining the first and second signals to provide a superheat signal which varies as a function of the temperature difference between the inlet and outlet portions of the evaporator; second circuit, comprising an adjusting means for establishing a temperature set point signal, a third sensor disposed in the controlled space to provide a third signal which varies as a function of the actual space temperature, and means for combining the set signal and the third signal to provide a control signal which varies as a function of the difference in temperature denoted by the set point signal I and the third signal; and

an all-electrical comparator stage, having a first input connection for receiving the superheat signal, a second input connection for receiving the control signal, and an output connection to provide the modulating signal required to regulate compressor operation and control the temperature of the space as a function of the difference between the superheat and control signals.

2. A control system as claimed in claim 1, and further comprising a resistor coupled in series with a capacitor between the first input connection and the output connection of the comparator stage, such that said capacitor affords an integrating function to stabilize the refrigeration control system, and a unidirectional current conduction means coupled in parallel with said resistor, connected in a sense to afford rapid charging of said capacitor as the superheat value decreases and the amplitude of the superheat signal increases by effectively bypassing said resistor, thus providing a capacity increase change rate of the entire control system which is substantially less than the capacity decrease change rate of the system.

3. A control system as claimed in claim 1 and further comprising a trigger circuit, having a first input connection connected to receive the signal related to the evaporator coil inlet temperature, a second input connection connected to receive a reference signal, and an output connection coupled to said first input connection of the comparator stage, to effectively override the superheat signal during start-up when there is no temperature difference across the evaporator coil.

4. A control system as claimed in claim 1 and further comprising a limiter stage, having a first input connection connected to receive said control signal, a second input connection for receiving a reference signal, and an output connection for applying an override signal to the second input connection of the signal combining means in the second circuit, to override the control signal under predetermined conditions and allow a zero capacity output indication when the superheat signal indicates zero superheat.

5. A refrigeration system including an evaporator, a compressor, a condenser and an expansion device, all intercoupled in a closed vapor cycle refrigeration circuit, a motor coupled to the compressor, and an inverter connected to energize the motor, with a control system for providing a modulating signal to regulate inverter operation, which control system comprises:

a first circuit, including a first thermistor positioned adjacent the evaporator inlet to provide a first signal which varies as a function of the evaporator inlet temperature, a second thermistor positioned adjacent the evaporator outlet to provide a second signal which varies as the evaporator outlet temperature, and a first operational amplifier connected to combine the first and second signals to provide a superheat signal which varies as a function of the temperature difference between the inlet portions of the evaporator;

a second circuit, comprising a potentiometer for establishing a temperature set point signal,- a third thermistor disposed in the controlled space to provide a third signal which varies as a function of the actual space temperature, and a second operational amplifier connected to combine the set point signal and the third signal to provide a control signal which varies as a function of the difference in temperatures denoted by the set point signal and the third signal; and

a comparator stage, including a third operational amplifier having a first input connection for receiving the superheat signal, a second input connection for receiving the control signal, and an output connection to provide the modulating signal required to regulate inverter operation and control the temperature of the space as a function of the difference between the superheat and control signals.

6. A control system as claimed in claim 5, and further comprising a resistor and capacitor coupled in series between the first input connection and the output connection of the third operational amplifier, such that said capacitor affords an integrating function to stabilize the refrigeration control system, and a diode coupled in parallel with said resistor, which diode is connected in a sense to afford rapid charging of said capacitor as the level of superheat decreases and the amplitude of the superheat signal increases by effectively bypassing said resistor, thus providing a capacity decrease change rate of the entire control system when the superheat decreases which is substantially greater than the capacity increase change rate of the control system when the superheat increases.

7. A control system as claimed in claim 5 and further comprising a trigger circuit, including a fourth operational amplifier having a first input connection connected to receive the signal related to the evaporator coil inlet temperature, a second input connection connected to receive a reference signal, and an output connection coupled to said first input connection of the third operation amplifier, to effectively override the superheat signal during start-up when there is no temperature difference across the evaporator coil.

8. A control system as claimed in claim 5 and further comprising a limiter stage, comprising a fifth operational amplifier having, a first input connection connected to receive said control signal, a second potentiometer connected in a voltage divider circuit to provide a reference signal, a second input connection for receiving said reference signal, and an output connection for applying an override signal to the second input connection of the second operational amplifier, to override the control signal under predetermined conditions and allow a zero capacity output indication when the superheat signal indicates zero superheat.

Patent No. Dated April 16 197 ofl Thomae Ca Jedimzzm et a1 are in the above-identifiec' patent eby corrected as shown below:

' t certified that erre':'a"'* and that said Letters Patent are hecolurrm 9, line 5, after =inlet insert and outlet? a iigned endsealed 12121.5 10th day 011 Sepfember 197 i.

MCCOY I La GIBSGN, J5. C MfibrmLL DAMN ttes'ting Officer Commissioner of Patents 

1. A refrigeration system including an evaporator, a compressor, a condenser and an expansion device, all intercoupled in a closed vapor cycle refrigeration circuit, with a control system for providing a modulating signal to regulate compressor operation, which control system comprises: a first circuit, including a first sensor positioned adjacent the evaporator input to provide a first signal which varies as a function of the evaporator inlet temperature, a second sensor positioned adjacent the evaporator output to provide a second signal which varies as the evaporator outlet temperature, and means for combining the first and second signals to provide a superheat signal which varies as a function of the temperature difference between the inlet and outlet portions of the evaporator; a second circuit, comprising an adjusting means for establishing a temperature set point signal, a third sensor disposed in the controlled space to provide a third signal which varies as a function of the actual space temperature, and means for combining the set signal and the third signal to provide a control signal which varies as a function of the difference in temperature denoted by the set point signal and the third signal; and an all-electrical comparator stage, having a first input connection for receiving the superheat signal, a second input connection for receiving the control signal, and an output connection to provide the modulating signal required to regulate compressor operation and control the temperature of the space as a function of the difference between the superheat and control signals.
 2. A control system as claimed in claim 1, and further comprising a resistor coupled in series with a capacitor between the first input connection and the output connection of the comparator stage, such that said capacitor affords an integrating function to stabilize the refrigeration control system, and a unidirectional current conduction means coupled in parallel with said resistor, connected in a sense to afford rapid charging of said capacitor as the superheat value decreases and the amplitude of the superheat signal increases by effectively bypassing said resistor, thus providing a capacity increase change rate of the entire control system which is substantially less than the capacity decrease change rate of the system.
 3. A control system as claimed in claim 1 and further comprising a trigger circuit, having a first input connection connected to receive the signal related to the evaporator coil inlet temperature, a second input connection connected to receive a reference signal, and an output connection coupled to said first input connection of the comparator stage, to effectively override the superheat signal during start-up when there is no temperature difference across the evaporator coil.
 4. A control system as claimed in claim 1 and further comprising a limiter stage, having a first input connection connected to receive said control signal, a second input connection for receiving a reference signal, and an output connection for applying an override signal to the second input connection of the signal combining means in the second circuit, to override the control signal under predetermined conditions and allow a zero capacity output indication when the superheat signal indicates zero superheat.
 5. A refrigeration system including an evaporator, a compressor, a condenser and an expansion device, all intercoupled in a closed vapor cycle refrigeration circuit, a motor coupled to the compressor, and an inverter connected to energize the motor, with a control system for providing a modulating signal to regulate inverter operation, which control system comprises: a first circuit, including a first thermistor positioned adjacent the evaporator inlet to provide a first signal which varies as a function of the evaporator inlet temperature, a second thermistor positioned adjacent the evaporator outlet to provide a second signal which varies as the evaporator outlet temperature, and a first operational amplifier connected to combine the first and second signals to provide a superheat signal which varies as a function of the temperature difference between the inlet portions of the evaporator; a second circuit, comprising a potentiometer for establishing a temperature set point signal, a third thermistor disposed in the controlled space to provide a third signal which varies as a function of the actual space temperature, and a second operational amplifier connected to combine the set point signal and the third signal to provide a control signal which varies as a function of the difference in temperatures denoted by the set point signal and the third signal; and a comparator stage, including a third operational amplifier having a first input connection for receiving the superheat signal, a second input connection for receiving the control signal, and an output connection to provide the modulating signal required to regulate inverter operation and control the temperature of the space as a function of the difference between the superheat and control signals.
 6. A control system as claimed in claim 5, and further comprising a resistor and capacitor coupled in series between the first input connection and the output connection of the third operational amplifier, such that said capacitor affords an integrating function to stabilize the refrigeration control system, and a diode coupled in parallel with said resistor, which diode is connected in a sense to afford rapid charging of said capacitor as the level of superheat decreases and the amplitude of the superheat signal increases by effectively bypassing said resistor, thus providing a capacity decrease change rate of the entire control system when the superheat decreases which is substantially greater than the capacity increase change rate of the control system when the superheat increases.
 7. A control system as claimed in claim 5 and further comprising a trigger circuit, including a fourth operational amplifier having a first input connection connected to receive the signal related to the evaporator coil inlet temperature, a second input connection connected to receive a reference signal, and an output connection coupled to said first input connection of the third operation amplifier, to effectively override the superheat signal during start-up when there is no temperature difference across the evaporator coil.
 8. A control system as claimed in claim 5 and further comprising a limiter stage, comprising a fifth operational amplifier having a first input connection connected to receive said control signal, a second potentiometer connected in a voltage divider circuit to provide a reference signal, a second input connection for receiving said reference signal, and an output connection for applying an override signal to the second input connection of the second operational amplifier, to override the control signal under predetermined conditions and allow a zero capacity output indication when the superheat signal indicates zero superheat. 